My Hope Vision LED is not broken.
Yesterday I thought it would not be wise to leave it attached to the bike, even in a rack pack, while parked in the city, so put it in a bag and took it with me. And then dropped it on a hard shop* floor. Twice. Heavily. So I checked it and found that it turned on but immediately turned itself off again. Oh dear, cacky coffee here! When I got home I tried it with fresh batteries and - lo! - it works properly.
The last time I'd used it before was on flashing mode in fog, where the batteries could have died without my noticing. And they have been in there a year, IIRC!
*Actually it was a cafe, where I had a very tasty piece of banana cake, so that was good!